If you’re unfamiliar with the concept of endosymbiosis, you’re in for a treat. If you know about it already, I hope you’ll still give the piece a shot because it should contain a couple of surprises.

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At first glance, a tree could not be more different from the caterpillars that eat its leaves, the mushrooms sprouting from its bark, the grass growing by its trunk, or the humans canoodling under its shade. Appearances, however, can be deceiving. Zoom in closely, and you will see that these organisms are all surprisingly similar at a microscopic level. Specifically, they all consist of cells that share the same basic architecture.

These cells contain a central nucleus—a command center that is stuffed with DNA and walled off by a membrane. Surrounding it are many smaller compartments that act like tiny organs, carrying out specialized tasks like storing molecules or making proteins. Among these are the mitochondria—bean-shaped power plants that provide the cells with energy.

This combination of features is shared by almost every cell in every animal, plant, fungus, and alga, a group of organisms known as “eukaryotes.”

Bacteria showcase a second, simpler way of building a cell—one that preceded the complex eukaryotes by at least a billion years. These “prokaryotes” always consist of a single cell, which is smaller than a typical eukaryotic one and bereft of internal compartments like mitochondria and a nucleus. Even though limited to a relatively simple cell, bacteria are impressive survival machines. They colonize every possible habitat, from miles-high clouds to the deep ocean. They have a dazzling array of biological tricks that allow them to cause diseases, eat crude oil, conduct electric currents, draw power from the Sun, and communicate with each other.

Still, without the eukaryotic architecture, bacteria are forever constrained in size and complexity. Sure, they have their amazing skill sets, but it’s the eukaryotes that cover the Earth in forest and grassland, that navigate the planet looking for food and mates, that build rockets to Mars.

The transition from the classic prokaryotic model to the deluxe eukaryotic one is arguably the most important event in the history of life on Earth. And in more than 3 billion years of existence, it happened exactly once.

Life is full of complex structures that evolve time and again. Individual cells have united to form many-celled creatures like animals and plants on dozens of separate occasions. The same is true for eyes, which have independently evolved time and again. But the eukaryotic cell is a one-off innovation.

Bacteria have repeatedly nudged along the path towards complexity. Some are very big (for microbes); others move in colonies that behave like single, many-celled creatures. But none of them have acquired the full suite of crucial features that define eukaryotes: large size, the nucleus, internal compartments, mitochondria, and more. As Nick Lane from University College London writes, “Bacteria have made a start up every avenue of eukaryotic complexity, but then stopped short.” Why?

It is not for lack of opportunity. The world is swarming with countless prokaryotes that evolve at breathtaking rates. Even so, they were not quick about inventing eukaryotic cells. Fossils tell us that the oldest bacteria arose between 3 and 3.5 billion years ago, but there are no eukaryotes from before 2.1 billion years ago. Why did the prokaryotes remain as simple cells for so damn long?

There are many possible explanations, but one of these has recently gained a lot of ground. It tells of a prokaryote that somehow found its way inside another, and formed a lasting partnership with its host. This inner cell—a bacterium—abandoned its free-living existence and eventually transformed into the mitochondria. These internal power plants provided the host cell with a bonanza of energy, allowing it to evolve in new directions that other prokaryotes could never reach.

If this story is true, and there are still those who doubt it, then all eukaryotes—every flower and fungus, spider and sparrow, man and woman—descended from a sudden and breathtakingly improbable merger between two microbes. They were our great-great-great-great-…-great-grandparents, and by becoming one, they laid the groundwork for the life forms that seem to make our planet so special. The world as we see it (and the fact that we see it at all; eyes are a eukaryotic invention) was irrevocably changed by that fateful union—a union so unlikely that it very well might not have happened at all, leaving our world forever dominated by microbes, never to welcome sophisticated and amazing life like trees, mushrooms, caterpillars, and us.

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8 thoughts on “The Greatest Merger In The History Of Life On Earth”

Have eyes really evolved independently over and over? We recently saw a gene that promotes formation of an eye in both vertebrates and arthropods, implying it had that role in the common ancestor of both. That common ancestor is right at the root of the animal kingdom. The eye was invented very early. It was then lost in many lineages, and once lost, apparently not re-invented. Eye design varies radically, but all modern eyes seem to come from that one eye eons back.

The quick invention of the original eye, and the failure to re-invent it in later eons, is a mystery. Let me suggest that in any environment where an eye would useful, some other critter that already has one has an overwhelming advantage over another that is newly experimenting with a proto-eye. A similar process might explain why mitochondria appear to have happened only once: subsequent re-occurrences happened in a world filled with eukaryotes that were much better at being eukaryotes than any new chimera could be.

You argue that endosymbiosis is “a freakishly improbable merger that happened once, and only once, in the history of the planet.” Have you considered the chloroplast? It’s my understanding that that had to have been a second endosymbiotic event similar to acquiring mitochondria.

It is first endosymbiotic event that is considered improbable, Diana. There have been multiple endosymbiotic events (more than one in fact that produced chloroplasts) after that. But all those others occurred with host cells capable of phagocytosis. If you can phagocytosis, ie swallow other cells whole, endosymbiosis is easy. But phagocytosis is a process that requires a lot of energy, and Martin and others’ argument is that it could have evolved if mitochondria are already present. That would mean that the first endosymbiotic event had to occur in a host cell without phagocytosis, and that is what makes it rare and unlikely.

Positing a creator explains nothing. All you do is replace the phrase “I don’t know” with “Creator did it”, “Creator chose this”, “Creator wanted it this way”, and that tells you nothing useful whatsoever. It does not explain why or how, or why things are this way in reality but not another way which a creator could also have created, and crucially, it provides no way of developing testable hypotheses to find out.

And then of course, you still have to explain where the creator came from.

Agreed. Some people will tell you that science deals with the “how” questions and religion deals with ther “why” questions.

But the distinction between the two is not always clear. Religion wonders why God is punishing us with a plague. Science explains how germs and genes explain the bulk of human illness, and eventually gets around to wiping out smallpox.

Ah, but there are the “ultimate why” questions. Aren’t there?
Saying “God did it” tells us who, not why. And if they could tell us how God did it, we would probably be able to dispense with the who. This, in a nutshell, is the history of science.

About Ed Yong

Ed Yong is a staff science writer at The Atlantic. His work has appeared in Wired, the New York Times, Nature, the BBC, New Scientist, Scientific American, the Guardian, the Times, and more. His first book I CONTAIN MULTITUDES—about how microbes influence the lives of every animal, from humans to squid to wasps—will be published in 2016 by Ecco (HarperCollins; USA) and Bodley Head (Random House; UK).

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